Position and orientation tracking systems (“trackers”) are well known in the art. For example, U.S. Pat. Nos. 4,287,809 and 4,394,831 to Egli et al.; U.S. Pat. No. 4,737,794 to Jones; U.S. Pat. No. 4,314,251 to Raab; and U.S. Pat. No. 5,453,686 to Anderson, are directed to AC electromagnetic trackers. U.S. Pat. No. 5,645,077 to Foxin discloses an inertial system, and combination systems consisting of two different trackers, such as optical and magnetic, are described in U.S. Pat. No. 5,831,260 to Hansen and U.S. Pat. No. 6,288,785 B1 to Frantz et al. Other pertinent references include U.S. Pat. No. 5,752,513 to Acker et al. and U.S. Pat. No. 5,640,170 to Anderson.
In the classical AC magnetic tracking system there typically is a single, static source of the three-axis fields which can be detected by multiple sensors which are free to move about a nearby volume (FIG. 1). Past magnetic systems wishing to cover more distance have created a larger source and driven it at high energy levels and then often even enlarged on that. This approach (see FIG. 2) always has proved difficult since the magnetic near field drops off as the third order of range from the source. That is, the signal is proportional to 1/r3.
Another factor is the error signal caused by magnetic signals creating responses that distort data because of eddy currents induced in nearby conductive materials. Although there is controversy over whether distortion is less or greater for pulsed DC or for AC magnetic trackers, in general there is very little difference if the objective is to obtain updates of tracking data very rapidly where stretching of the pulsed DC cycle to allow transients to decay prior to data collection is not allowed.
Although the desired direct magnetic signal and the eddy current distortion signal in theory maintain a constant ratio with energy level, there is a nonlinear phenomenon which alters this constant ratio. When operating at or above the signal level where the nonlinearity occurs, proportionality holds. Consequently, increasing source drive in order to increase operating range creates no benefit over most of the volume because distortion continues as a serious problem. Hence, a large magnetic field source is quite limited in extending operating range. Reversal of the source and sensor roles here offers an alternative for covering a larger volume.
If the source drive level is kept low such that the effects of secondary fields from eddy currents tends to fall at or below the noise floor of the sensing circuitry, that is the source-sensor coupling range is kept short, distortion is rarely a significant problem. In short, the nonlinearity of the noise floor acts as a natural “filter” against the weaker eddy current fields, which must cover much more distance to where the eddy currents are generated and onward to the sensor than does the direct signal. Therefore, if we were to distribute multiple sensors along the periphery of a volume that exceeds the normal source-sensor operating range, then a small, low power source acting as a “sensor” offers the opportunity to track an object over a large volume (see FIG. 3) without eddy current distortion being a derogatory factor.
Of course, operation of several static sensors in order to track a source pseudo-“sensor” raises the issue of maintaining several movement reference points in the volume. That is, there can be one at each sensor. The track of position and orientation (P&O) reported out to the host computer must be referenced to a common point. This point could be one of the sensors or some arbitrary point known by the system. Fortunately, referencing movement back to a common point is a relatively simply geometry problem with somewhat more complex bookkeeping of the various known sensor data points and the computation of track data. The benefits that make this worthwhile are avoidance of raising eddy current distortion and still maintaining strong signals throughout a large volume.
What makes this tracking over a larger area difficult is the incoherency of signal frequencies between a remote wireless source and the tracking sensor(s). Tracking of both regimes of sensors from a source and sources from one or more sensors (FIG. 4) has been done for many years as long as they are connected to a single set of electronics. However, existing systems do not provide the freedom to move through a 3D volume with or without being wired.
Initial landmark AC tracker literature made no distinction between whether the source or the sensors were static or moving. It simply states that the position and orientation (P&O) reported was the P&O relative to each other. In some later disclosures the concept of making the source(s) move and leaving the sensor(s) static was given innovative stature nevertheless. However, the systems cited remained tethered through cabling and greatly simplified the engineering problem of signal detection, synchronization and tracking.
The advent of microcircuits improved battery longevity and more sensitive receiving circuitry as well as providing significantly more cost effective processing. This has made possible wireless field sources which can generate detectable signals of sufficient strength for tracking and do so for at least an hour before battery re-charging. The consequence of this situation is that small 3-axis field sources now offer a way to achieve wireless P&O tracking without the need of radio links if on-the-fly signal detection and synchronization can be provided for small wireless field sources.
Several previous patents deal with tracking the movement of passive sensors relative to a stationary source of AC magnetic fields. U.S. Pat. No. 4,054,881 to Raab is one example, Tracking of remote sources with sensors is one subject of U.S. Pat. No. 6,188,355 to Gilboa. Gilboa also discusses the source being wireless under several constraints for achieving synchronization between the source signals and the sensors. In one embodiment there is a requirement to switch the wireless source and the tracking sensors back and forth between transmit and receive in order to obtain synchronization between them. In another embodiment there is a requirement that the three frequencies generated, one for each leg of the transmitting coil, be harmonically related. In yet another embodiment reception of a threshold triggering event at the wireless source in order to start all transmitted signals in unison is explained. These constraints, plus a requirement to perform calibrations at over 32 position and 32 orientation settings, leads to significant complexity, considering that phase adjustments are subject to drift over time.